Manufacturing Processes for Engineering Materials 5th Edition in SI Units

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Manufacturing Processes for Engineering
Materials (5th Edition in SI Units)

• Chapter 11
• Properties and Processing of
• Metal Powders, Ceramics, Glasses and
  Superconductors
• Powder metallurgy (PM, ?? ??)
• Ceramics
• Glasses

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Powder Metallurgy (????)

• Powder-metallurgy operation consists of the
  following sequence steps
• Powder production
• Blending
• Compaction
• Sintering
• Finishing operations

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Making Powder-Metallurgy Parts
Figure extra Outline of processes and operations
involved in making powder-metallurgy parts.
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Powder Metallurgy

• Finishing operations include
• Coining
• Sizing
• Machining
• Infiltration for improved quality
• Dimensional accuracy
• Part strength

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Production of metal powders

• The shape, size distribution, porosity, chemical
  purity, bulk and surface characteristics of the
  powder particles depend on the particular
  processes used.

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Production of metal powders

• The methods of powder production are
• 1. Atomization
• Produces liquid-metal stream by injecting molten
  metal.
• 2. Reduction
• Removal of oxygen involves gases such as reducing
  agents.
• 3. Electrolytic deposition
• Uses either aqueous solutions or fused salts.
• 4. Carbonyls
• 5. Comminution

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Particle size, distribution and shape

• Particle size is measured and controlled by
  screening.
• Other methods are also used for particle size
  analysis
• Sedimentation
• Microscopic analysis
• Light scattering
• Optical means
• Suspension of particles
• Size distribution of particles affects the
  processing characteristics of the powder.

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Particle size, distribution and shape

• Particle shape influences on processing
  characteristics.
• Shape is described in terms of aspect ratio or
  shape index.
• Aspect ratio is the ratio of the largest
  dimension to the smallest dimension of the
  particle.
• Shape index, or shape factor (SF), is a measure
  of surface area to the volume of a particle.

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Example 11.1Particle shape-factor determination

• Determine the shape factor for a (a) spherical
  particle, (b) cubic particle, and
• (c) cylindrical particle with a
  length-to-diameter ratio of 2.
• Solution
• The ratio of surface area to volume is
• The shape factor is

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Example 11.1Particle shape-factor determination

• Solution
• The surface area of a cylindrical particle with
  length-to-diameter ratio of 2 is
• The particles volume is
• Equivalent diameter is
• Shape factor is

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Blending metal powders

• Blending (mixing) powders is the second step, and
  the purposes are
• to impart physical and mechanical properties and
  characteristics to the P/M part
• obtain uniformity from part to part
• lubricants are mixed to improve flow
  characteristics
• additives used to facilitate sintering

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Compaction of Metal Powders

• Compaction are pressed into shapes using dies and
  presses.
• Obtain the required shape, density and
  particle-to-particle contact.

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Density Variations in Dies
Figure 11.8 Density variation in compacting
metal powders in various dies (a) single-action
press (b), (c) and (d) double-action press.
Note in (d) the greater uniformity of density,
from pressing with two punches with separate
movements, compared with (c). (e) Pressure
contours in compacted copper powder in a
single-action press. Source P. Duwez and L.
Zwell.
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Compaction of Metal Powders

• The density after compaction (green density)
  depends on
• compaction pressure
• powder composition
• hardness of the powder
• Higher the density, higher the strength and
  elastic modulus of the part.

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Pressure distribution in powder compaction

• Pressure distribution along length of the compact
  can analyse by slab method.
• Balancing the vertical forces,
• k is a measure of the interparticle friction
  during compaction,

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Pressure distribution in powder compaction

• When there is no friction between the particles,
• k 1,
• Noting that the boundary condition
• Thus, as pressure decays as coefficient of
  friction, k and length-to-diameter ratio increase.

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Example 11.3Pressure decay in compaction

• Assume that a powder mix has the values of k
  0.5 and µ 0.3. At what depth will the pressure
  in a straight cylindrical compact 10 mm in
  diameter become (a) zero and (b) one-half the
  pressure at the punch?
• Solution
• When px 0,
• Note when x approach infinity, pressure will
  decay to 0.
• When px/p0 0.5,
• 50 pressure drop is severe as compact density
  will be unacceptably low.

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Isostatic pressing

• Powders are subjected to hydrostatic pressure in
  order to to achieve uniform compaction.
• In cold isostatic pressing (CIP), metal powder is
  placed in a flexible rubber mold.

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Isostatic pressing

• In hot isostatic pressing (HIP), a container is
  made of high-melting point sheet metal and the
  pressurizing medium is an inert gas.
• It can produce compacts with uniform grain
  structure and density, irregardless of shape.

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Sintering

• Sintering is where compacted metal powder is
  heated to below its melting point for the bonding
  of the individual metal particles.
• Density of a sintered part depends on a parts
  green density, temperature, time and furnace
  atmosphere.
• Sintered density increases with temperature and
  time.

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Sintering

• Sintering mechanisms
• Sintering mechanisms depend on composition of
  metal particles and processing parameters.
• Question Is the melting temp. constant? or Does
  it depend on size?
• 1st mechanism
• As temperature increases, 2 particles will bond
  by diffusion. (T lt Tm)
• Strength, density, ductility, thermal and
  electrical conductivities of the compact also
  increase.
• At the same time, allowances are needed for
  compact shrink.

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Sintering

• Sintering mechanisms
• 2nd mechanism
• Vapor-phase transport is due to material heated
  close to melting temperature which releases metal
  atoms to the vapor phase.
• Depending on temperature, time and processing
  history, different structures and porosities can
  be obtained.

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Example 11.4Shrinkage in sintering
In solid-state bonding during sintering of a
powder-metal green compact, the linear shrinkage
is 4. If the desired sintered density is 95 of
the theoretical density of the metal, what should
be the density of the green compact? Ignore
the small changes in mass that occur during
sintering. Solution Volume shrinkage during
sintering is Mass does not change during
sintering, Thus,
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Economics of Powder Metallurgy

• The cost depends on method of powder production,
  its quality and quantity purchased.
• Due to high cost of punches, dies and equipment
  for P/M processing, production volume must be
  high.
• P/M forging is used for critical applications
  where full density and fatigue resistance are
  essential.

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Ceramics Structure, Properties,and Applications

• Ceramics are compounds of metallic and
  non-metallic elements.

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Structure and types of ceramics

• Structure of ceramic crystals contains elements
  of different sizes.
• Bonding between atoms can be covalent and ionic.
• Various types of ceramics are
• 1. Oxide ceramics
• - Alumina
• - Zirconia
• 2. Other ceramics
• - Carbides
• - Nitrides

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General properties and applications of ceramics

• Ceramics are brittle, have high compressive
  strength and hardness at elevated temperatures,
  high elastic modulus, low toughness, low density,
  low thermal expansion, and low thermal and
  electrical conductivity.
• 1. Mechanical properties
• Sensitivity to cracks, impurities and porosity
• Strength in tension is lower than compressive
  strength.
• 2. Physical properties
• Low specific gravity and have high melting
  temperatures.

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Example 11.7Effect of porosity on properties
If a fully dense ceramic has the properties of
UTS0 100 MPa, E0 400 GPa, and K0 0.5 W/m-K,
what are these properties at 10 porosity? Assume
that n 5. Solution We have The modulus of
elasticity is The thermal conductivity, k, is
related to porosity by
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Shaping Ceramics

• Ceramics can be shaped into useful products.
• Procedure involves (??? ?? ?? ??)
• crushing raw materials into very fine particles
• mixing particles with additives
• shaping, drying and firing the material

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Example 11.9Dimensional changes during shaping
of ceramic components
A solid cylindrical ceramic part is to be made
whose final length must be L 20mm. It has been
established that for this material, linear
shrinkages during drying and firing are 7 and
6, respectively, based on the dried dimension.
Calculate (a) the initial length of the part and
(b) the dried porosity if the porosity of the
fired part is 3. Solution a) Since firing is
preceded by drying, Also, b) Since final
porosity is 3, Therefore, the porosity of the
dried part is 19.
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Glasses Structure, Properties,and Applications

• Glass is an amorphous solid with the structure of
  a liquid.
• All glasses contain at least 50 silica.
• They are resistant to chemical attacks and ranked
  by their resistance to acid, alkali or water
  corrosion.

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Mechanical properties

• Consider to be linearly elastic and brittle.
• Range of elastic modulus is 5590 GPa and
  Poissons ratio ranges from 0.160.28.
• Have low strength due to presence of small flaws
  and microcracks on the surface of the glass.

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Physical properties

• Low thermal conductivity and high dielectric
  strength.
• Thermal expansion coefficient is lower than
  metals and plastics.
• Optical properties can be modified by varying
  their composition and treatment.

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Forming and Shaping Glass

• Glass products are categorized as
• Flat sheet or plate
• Rods and tubing
• Discrete products
• Glass fibers
• ??? ????

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Graphite

• Graphite has a set of close-packed carbon atoms.
• It is brittle, has high electrical, high
  temperature, thermal conductivity and resistance
  to thermal shock.
• Resistance to chemicals, low frictional
  properties allow it to be a solid lubricant,
  abrasive and a poor lubricant in a vacuum.
• Graphite is graded in terms of decreasing order
  of grain size.

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Diamond

• Has a covalently bonded structure and is the
  hardest substance.
• It is brittle and decompose in air at about 973
  K.
• Has superior properties because of its lack of
  impurities.
• Applications include cutting tools, razors for
  shaving, and high-performance automotive engine
  components.

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Metal-matrix composites

• The advantage is it has higher resistance to
  elevated temperatures and higher ductility and
  toughness.
• Limitations are that it has higher density and
  makes processing more difficult.